1. Field of the Invention
The invention relates generally to rigid porous carbon structures. More specifically, the invention relates to rigid three dimensional structures comprising carbon nanofibers and having high surface area and porosity, low bulk density, low amount of micropores and increased crush strength and to methods of preparing and using such structures. The invention also relates to using such rigid porous structures for a variety of purposes including catalyst supports, electrodes, filters, insulators, adsorbents and chromatographic media and to composite structures comprising the rigid porous structures and a second material contained within the carbon structures.
2. Description of the Related Art
Heterogeneous catalytic reactions are widely used in chemical processes in the petroleum, petrochemical and chemical industries. Such reactions are commonly performed with the reactant(s) and product(s) in the fluid phase and the catalyst in the solid phase. In heterogeneous catalytic reactions, the reaction occurs at the interface between phases, i.e., the interface between the fluid phase of the reactant(s) and product(s) and the solid phase of the supported catalyst. Hence, the properties of the surface of a heterogeneous supported catalyst are significant factors in the effective use of that catalyst. Specifically, the surface area of the active catalyst, as supported, and the accessibility of that surface area to reactant chemisorption and product desorption are important. These factors affect the activity of the catalyst, i.e., the rate of conversion of reactants to products. The chemical purity of the catalyst and the catalyst support also have an important effect on the selectivity of the catalyst, i.e., the degree to which the catalyst produces one product from among several products, and the life of the catalyst.
Generally catalytic activity is proportional to catalyst surface area. Therefore, high specific area is desirable. However, that surface area must be accessible to reactants and products as well as to heat flow. The chemisorption of a reactant by a catalyst surface is preceded by the diffusion of that reactant through the internal structure of the catalyst and the catalyst support, if any. The catalytic reaction of the reactant to a product is followed by the diffusion of the product away from the catalyst and catalyst support. Heat must be able to flow into and out of the catalyst support as well.
Since the active catalyst compounds are often supported on the internal structure of a support, the accessibility of the internal structure of a support material to reactant(s), product(s) and heat flow is important. Porosity and pore size distribution of the support structure are measures of that accessibility. Activated carbons and charcoals used as catalyst supports have surface areas of about 1000 square meters per gram and porosities of less than one milliliter per gram. However, much of this surface area and porosity, as much as 50%, and often more, is associated with micropores, i.e., pores with pore diameters of 2 nanometers or less. These pores can be difficult to access because of diffusion limitations. Moreover, they are easily plugged and thereby deactivated. Thus, high porosity materials where the pores are mainly in the mesopore (&gt;2 nanometers) or macropore (&gt;50 nanometers) ranges are most desirable.
It is also important that supported catalysts not fracture or attrit during use because such fragments may become entrained in the reaction stream and must then be separated from the reaction mixture. The cost of replacing attritted catalyst, the cost of separating it from the reaction mixture and the risk of contaminating the product are all burdens upon the process. In other processes, e.g. where the solid supported catalyst is filtered from the process stream and recycled to the reaction zone, the fines may plug the filters and disrupt the process.
It is also important that a catalyst, at the very least, minimize its contribution to the chemical contamination of reactant(s) and product(s). In the case of a catalyst support, this is even more important since the support is a potential source of contamination both to the catalyst it supports and to the chemical process. Further, some catalysts are particularly sensitive to contamination that can either promote unwanted competing reactions, i.e., affect its selectivity, or render the catalyst ineffective, i.e., "poison" it. Charcoal and commercial graphites or carbons made from petroleum residues usually contain trace amounts of sulfur or nitrogen as well as metals common to biological systems and may be undesirable for that reason.
While activated charcoals and other carbon-containing materials have been used as catalyst supports, none have heretofore had all of the requisite qualities of porosity and pore size distribution, resistance to attrition and purity for use in a variety of organic chemical reactions. For example, as stated above, although these materials have high surface area, much of the surface area is in the form of inaccessible micropores (i.e., diameter &lt;2 nm).
Nanofiber mats, assemblages and aggregates have been previously produced to take advantage of the high carbon purities and increased accessible surface area per gram achieved using extremely thin diameter fibers. These structures are typically composed of a plurality of intertwined or intermeshed fibers. Although the surface area of these nanofibers is less than an aerogel or activated large fiber, the nanofiber has a high accessible surface area since the nanofibers are substantially free of micropores.
One of the characteristics of the prior aggregates of nanofibers, assemblages or mats made from nanofibers is low mechanical integrity and high compressibility. Since the fibers are not very stiff these structures are also easily compressed or deformed. As a result the size of the structures cannot be easily controlled or maintained during use. In addition, the nanofibers within the assemblages or aggregates are not held together tightly. Accordingly, the assemblages and aggregates break apart or attrit fairly easily. These prior mats, aggregates or assemblages are either in the form of low porosity dense compressed masses of intertwined fibers and/or are limited to microscopic structures.
Moreover, the above described compressibility of the nanofiber structures may increase depending on a variety of factors including the method of manufacture. For example, as suspensions of the nanofibers are drained of a suspending fluid, in particular water, the surface tension of the liquid tends to pull the fibrils into a dense packed "mat". The pore size of the resulting mat is determined by the interfiber spaces which, because of the compression of these mats, tend to be quite small. As a result, the fluid flow characteristics of such mats are poor.
Alternatively, the structure may simply collapse under force or shear or simply break apart. The above described nanofiber structures are typically two fragile and/or too compressible to be used in such products as fixed beds or chromatographic media. The force of the fluid flow causes the flexible assemblages, mats or aggregates to compress, otherwise restricting flow. The flow of a fluid through a capillary is described by Poiseuille's equation which relates the flow rate to the pressure differential, the fluid viscosity, the path length and size of the capillaries. The rate of flow per unit area varies with the square of the pore size. Accordingly, a pore twice as large results in flow rates four times as large. The presence of pores of a substantially larger size in a nanofiber structure results in increased fluid flow because the flow is substantially greater through the larger pores. Decreasing the pore size by compression dramatically reduces the flow. Moreover, such structures also come apart when subjected to shear resulting in the individual nanofibers breaking loose from the structure and be transported with the flow.
As set forth above, prior aggregates, mats or assemblages provide relatively low mechanical properties. Accordingly, although previous work has shown that nanofibers can be assembled into thin, membrane-like or particulate structures through which fluid will pass, such structures are flexible and compressible and are subject to attrition. Accordingly, when these structures are subjected to any force or shear, such as fluid or gas flow, these structures collapse and/or compress resulting in a dense, low porosity mass having reduced fluid flow characteristics. Moreover, although the individual nanofibers have high internal surface areas, much of the surface of the nanofiber structures is inaccessible due to the compression of the structure and resulting decrease in pore size.
It would be desirable to produce a rigid porous carbon structure having high accessible surface area, high porosity, increased rigidity and significantly free from or no micropores. This is particularly true since there are applications for porous carbon structures that require fluid passage and/or high mechanical integrity. The compressibility and/or lack of rigidity of previous structures of nanofibers creates serious limitations or drawbacks for such applications. The mechanical and structural characteristics of the rigid porous carbon structures brought about by this invention make such applications more feasible and/or more efficient.